The present invention relates to a method for analyzing the driving behavior of motor vehicles on a roller type test stand.
The target variables in the development and optimization of the drive system of motor vehicles are emission behavior, consumption, performance and driveability. In general, driveability is a subjective concept, in particular concerning the behavior of a vehicle in transient operational states as assessed by the driver. When the accelerator pedal of a car is depressed rapidly, the driver will feel more comfortable if acceleration sets in quickly and is jerk-free. The same is true for other transient operating conditions, such as a sudden shift into overrun or return from overrun. Any delays, irregularities or variations in vehicle response as a result of driver-induced changes are usually felt to be irritating. Examples include fore and aft oscillations (surge), delays in response, oscillations in engine speed or tractive power. Positive feelings are evoked by prompt accelerator response, smooth engine power, or quiet and stable idling. A vehicle""s driveability is significantly influenced by the engine management, but also by engine suspension and design of the power train as a whole. In view of current attempts to optimize fuel consumption and exhaust emissions by influencing engine management, maintaining or increasing driveabuilty must be considered as a further target variable.
It is a problem in this context that objective and reproducible criteria in determining a vehicle""s driveability are hard to be found in practice, compared with the determination of fuel consumption or exhaust emissions. A further disadvantage is that in the early stages of vehicle development test vehicles are not usually available, so that transient engine management functions must be optimized on a dynamic test stand. Such tests have not been successful so far in producing reliable ratings of a vehicle""s driveability.
In EP 0 846 945 A of the applicant, methods and apparatuses are described that enable the highest possible objective determination of the driveability. The vehicle is subjected in real driving operation to a large number of measurements, with the measurement of the longitudinal acceleration being of particular relevance because surge oscillations have a relevant influence on the driveability. It is also described in said printed publication how a forecast can be made on a dynamic test stand when there is a precise simulation model of the vehicle which reflects the power train in a detailed manner in particular. The production of such a simulation model is very complex, however.
Although the recording of data for determining the driveability in driving operations is easily possible, it is still desirable in many cases to simultaneously perform further measures such as measurements of noise or exhaust gas. In a number of cases the vehicle to be examined is moreover not in a condition that would simply allow driving on an open road. In such cases it is desirable to have the possibility to examine the driveability on a roller type test stand.
With the known methods it is not possible to perform a determination of the driveability, because signals on the longitudinal acceleration in particular are not available. However, they are relevant in the determination of the driveability. The production of a sufficiently precise model of the vehicle situated on the roller type test stand is very difficult, because it is also necessary to consider the dynamics of the test stand and the coupling of the vehicle with the rollers.
It is the object of the present invention to provide a simple and reliable method for determining the driveability of a motor vehicle which can be used on a roller type test stand.
The method is provided in accordance with the invention with the following steps:
Performance of a predetermined operating cycle on a motor vehicle located on a roller type test stand;
determination of a first longitudinal acceleration signal from a speed signal which is tapped from the power train of the motor vehicle or the roller type test stand;
determination of a second longitudinal acceleration signal by an acceleration sensor fastened to the vehicle;
superposition of the first and second longitudinal acceleration signal into a composite signal;
determination of a parameter from the composite signal which is representative of the driveability of the motor vehicle.
The relevant aspect of the invention is that the composite signal determined at the test stand by calculation can be used in the determination of the driveability in an analogous manner as the longitudinal acceleration determined in real driving operation. Methods can therefore be used as are described in EP 0 846 945 A insofar as the same relate to the observation of the real vehicle.
As in known methods, the vehicle is subjected to a test cycle, with trigger conditions corresponding to typical driving situations being acquired through the observation of different measured variables. By analyzing the measured variables during the occurrence of the trigger conditions, parameters can be determined which allow drawing conclusions on the driveability of the vehicle. Such methods are described in detail in EP 846 945 A. The present invention now also allows performing such methods on a vehicle which is located on a roller type test stand. This helps achieving a simplification of the test procedure and simultaneously performing additional measurements with simple means which relate to the emission of exhaust gases or the noise development for example.
It is particularly favorable if the first longitudinal acceleration signal is derived from a speed signal by differentiation. The engine speed can be used in this connection for example as the speed signal. The theoretical speed of the vehicle can be calculated by considering the transmission gear ratio in the respectively engaged gear, the transmission ratio of the driving axle and the tread of the tires. As an alternative, the directly determined speed of a wheel can be converted into a speed value, or said value is calculated from the speed of the rollers of the test stand. The theoretical longitudinal acceleration is obtained by the differentiation of the speed. This variable is not suitable for determining the driveability, because the characteristic surge frequencies are represented in a highly distorted way. A useful value is only obtained by the superposition in accordance with the invention with a longitudinal acceleration signal as tapped from the vehicle per se.
A distortion by artifacts can be minimized especially in such a way that the first longitudinal acceleration signal is subjected to a low-pass filtering. A cut-off frequency of between 1 and 2 Hz is suitable for most applications.
It is similarly favorable when the second longitudinal acceleration signal is subjected to a high-pass filtering, with the cut-off frequency being 0.5 to 1.5 Hz. It is favorable when the cut-off frequency of the high-pass filter lies slightly below the one of the low-pass filter. These filterings are adjusted to one another in such a way that the cut-off frequency of the filter is set to a value which ensures that the low-frequency share is taken to such an extent from the first longitudinal acceleration signal as it would correspond to the actual longitudinal acceleration on the road. The respective filtered share of the second longitudinal acceleration signal is taken from said frequency in order to represent the higher-frequency shares of the longitudinal acceleration in an undistorted way. It has been seen that a composite signal can be obtained by such a method which exceedingly conforms to a real longitudinal acceleration signal.
A further improvement in the precision of the conformity can be achieved in such a way that in the formation of the composite signal, the group delay times are corrected in the high-pass filter and/or low-pass filter. In this way the relative phase position of the individual frequency components can be brought into a relation as is present in the real signal.
A comprehensive evaluation of the driveability can be achieved in such a way that, in addition to the composite signal for the determination of a parameter representative of the driveability of the vehicle, at least one additional measuring variable is used from the following group: engine speed, position of throttle valve or gas pedal, vehicle speed, suction pipe vacuum, coolant temperature, ignition point, injection quantity, lambda value, exhaust gas recirculation value and exhaust gas temperature. The determination of a value for the vehicle speed is performed as described above. A correction of the higher-frequency shares is not required in this case because they are not included in the calculation of the driveability, apart from the longitudinal acceleration which is processed separately.
A particularly precise statement on the driveability can be made in such a way that the oscillation amplitude at characteristic surge frequencies is derived from the composite signal.
The invention further relates to an apparatus for the analysis of the driving behavior of motor vehicles on a roller type test stand, including a first sensor for detecting a speed signal from the power train of the motor vehicle or of the roller type test stand, an acceleration sensor for detecting the longitudinal acceleration of the motor vehicle, and an evaluation device which is connected with the first sensor and with the acceleration sensor and is arranged so as to further process the signals of the sensors and superimpose the same into a composite signal in order to calculate from the composite signal a parameter representative of the driveability of the motor vehicle. As was described above, such an apparatus provides a precise determination of the driveability on a roller type test stand.
It is particularly favorable when the acceleration sensor is disposed in the area of a neck-rest of the vehicle. It has been noticed that in this way a particularly precise conformity with the driver""s perceptions is achieved, particularly when the neck-rest concerns that of the driver""s seat.
As has already been described, it is advantageous when the evaluation device comprises a differentiator and a low-pass filter for the first longitudinal acceleration signal and/or a high-pass filter for the second longitudinal acceleration signal.